In addition to be applied for logic, memory and functional device circuits, the N-shaped negative-differential-resistance (NDR) device has been used as a special-application switch. To offer an effective switching action, a switch should have two stable operating states, the high-current, low-voltage "on-state" and the low-current, high-voltage "off-state"). The switch with these two stable operating states will have low off-state power dissipation, and a large on/off current ratio. Such performance can only be achieved from a low-impedance on-state and a high-impedance off-state. In general, a S-shaped NDR device is more suitable for use as a semiconductor switch in view of its low on-state impedance and high off-state impedance. However, most of the S-shaped NDR devices are of a PNPN multi-layer structure, and thus the fabrication process thereof is complicated and difficult.
For most conventional N-shaped NDR devices, such as tunneling diode and resonant tunneling diode, the off-state impedance is small. Hence, when acting as switches, they will suffer a low on/off current ratio and high power loss at the off-state due to a large off-state current. The on/off current ratio is determined by the peak-to-valley current ratio (PVCR) which is not high for most conventional N-shaped NDR devices. Therefore, the switching action is not obvious when the conventional N-shaped NDR devices are used as a switch. In addition, the selections of bias and load of the switches are restricted in a great extent. In order to enhance the PVCR, some N-shaped NDR devices were modified to have more complicated structures, for examples .delta.-doping layers, double quantum wells, tunneling devices and integrated transistors. However, these modified structures require more sophisticated fabrication techniques and thus increase the difficulty of fabrication.
The main objective of the present invention is to provide a N-shaped NDR semiconductor switch having a simple structure which is easy to be made, and an excellent switching performance similar to the S-shaped NDR semiconductor switch. More specifically, the N-shaped NDR semiconductor switch of the present invention will be devoid of the following drawbacks of the prior art:
1. the high off-state power dissipation and low on/off current ratio of the conventional N-shaped NDR semiconductor switch; PA1 2. the complicated structures of the conventional N-shaped NDR semiconductor switch and the conventional S-shaped NDR semiconductor switch, and the difficult fabrication processes thereof; and PA1 3. the relatively high price of the conventional N-shaped NDR semiconductor switch and the incompatibility with the fabrication processes of Si integrated circuits resulting from its III-V structure, and the problem of not able to work at high temperature (&gt;100.degree. C.). PA1 a) removing a native oxide on a single crystal silicon (Si) substrate doped with impurities of a first conductive type; PA1 b) forming a graded-composition layer on a surface of the Si substrate from the step a) in a reaction chamber by chemical vapor deposition, wherein a fixed flow rate of silane gas is introduced into the reaction chamber and a gradually increasing flow rate of alkane gas is introduced into the reaction chamber during the chemical vapor deposition, wherein the flow rate of the alkane gas is zero when the chemical vapor deposition is initiated; and PA1 c) forming a silicon carbide (SiC) layer doped with impurities of a second conductive type on the graded-composition layer.